Synthesis and Characterization of Blue Phosphorescent NHC-Ir(III) Complexes with Annulated Heterocyclic 1,2,4-Triazolophenanthridine Derivatives for Highly Efficient PhOLEDs

Efficient tris-bidentate Ir­(III)
phosphorescent dopants were prepared
using a series of 1,2,4-triazolo­[4,3-f]­phenanthridine
(tzp) moieties modified with aryl substituents (phenyl, tolyl, and
xylenyl) as the main phenylimidazole-based N-heterocyclic carbene
(NHC) chelates (C∧C:). According to the degree of
the bulkiness of the aryl substituent and the ligation mode, the five
prepared Ir­(tzpC∧C:) complexes include
four homoleptic NHC-Ir­(III) complexes, fac-Ir­(tzpPh)3, fac-/mer-Ir­(tzpTol)3, and mer-Ir­(tzpXyl)3, and
one heteroleptic NHC-Ir­(III) complex, cis-Ir­(tzpPh)2(tzpPh)′, in which the
phenyl moiety of one tzpPh ligand is abnormally ligated to the Ir
metal center, unlike other tzp ligands. The Ir­(III) complexes ligated
by carbene ligands (tzpC∧C:) exhibited
highly efficient emissions in the solid state (Φem = 23.2–54.0%). Electrochemical and theoretical studies revealed
that the excited-state properties of these NHC-Ir­(III) complexes are
variable on the extent of planarity and π-conjugation of the tzpC∧C: chelating ligand. Due to its enhanced
rigidity and low excited-state energy, a result of abnormal tzpPh
ligand ligation, the heteroleptic cis-Ir­(tzpPh)2(tzpPh)′ exhibited the
most efficient emission properties in solution (Φem = 21.4%) and solid (Φem = 54.0%) media. Of the
devices fabricated with Ir­(tzpC∧C:)3 complexes as emitters, that doped with cis-Ir­(tzpPh)2(tzpPh)′ exhibited superior electroluminescence efficiencies (external quantum
efficiency (EQE) of 16.3%, current efficiency of 27.6 cd A–1, and power efficiency of 22.1 lm W–1) and CIE
coordinates of [0.17,0.26], which are superior to those of other Ir­(tzpC∧C:)3 complexes and Ir­(dmp)3 (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo­[1,2-f]­phenanthridine). This study provides insight into the
molecular-level engineering of Ir­(III) dopant materials for improving
the emission efficiencies of phosphorescent OLEDs.